Lyme borreliosis or Lyme disease is a multisystem infectious disease caused by the spirochete Borrelia burgdorferi which is transmitted to humans by Ixodes ticks. It is the most common vector borne disease in the United States & Europe. While considerable information is available on the epidemiology, clinical course and acute and chronic manifestations of this epidemic disease. In recent studies we have made four observations which directly relate to the early steps in the evolution of the disease: a) there are major differences in the infectivity of the spirochete depending on the route of inoculation with the intradermal route being the most efficient for establishing an experimental infection, b) that cells like fibroblasts and keratinocytes, which are abundant at the skin inoculation site, protect B. burgdorferi from the cidal action of antibiotics which are active in the absence of these cells, c) B. burgdorferi can be localized within dermal fibroblasts by laser scanning confocal microscopy, and d) that strains of B. burgdorferi which are serially passaged in vitro and which become non-infectious in a mouse infectivity model coincidentally lose the ability to resist elimination by host phagocyte cells. These observations provide some of the first correlations between infectivity of the organism (virulence) and its interaction with a host defense system and host environments which are encountered by the spirochete during natural infection. The goals of this proposal are to investigate how fibroblasts, keratinocytes and skin organ culture protect B. burgdorferi, to determine whether interaction with fibroblasts, keratinocytes or skin organ culture induces new borrelial antigens or proteins which relate to infectivity of the spirochete and to define the mechanism(s) by which infectious B. burgdorferi evade phagocytic cells. Using direct and indirect methods we will try to gain insight into the location of B. burgdorferi within fibroblasts, study of surface components which facilitate the protective interaction of fibroblasts with borrelia against antibiotics, exam new protein synthesis and probe for new immunoreactive epitopes or borrelia exposed to the skin environment and compare the interactions of infectious and non-infectious spirochetes with human neutrophils and macrophages.